Secreted PCSK9 promotes LDL receptor degradation independently of proteolytic activity

Abstract

PCSK9 (proprotein convertase subtilisin/kexin 9) is a secreted serine protease that regulates cholesterol homoeostasis by inducing post-translational degradation of hepatic LDL-R [LDL (low-density lipoprotein) receptor]. Intramolecular autocatalytic processing of the PCSK9 zymogen in the endoplasmic reticulum results in a tightly associated complex between the prodomain and the catalytic domain. Although the autocatalytic processing event is required for proper secretion of PCSK9, the requirement of proteolytic activity in the regulation of LDL-R is currently unknown. Co-expression of the prodomain and the catalytic domain in trans allowed for production of a catalytically inactive secreted form of PCSK9. This catalytically inactive PCSK9 was characterized and shown to be functionally equivalent to the wild-type protein in lowering cellular LDL uptake and LDL-R levels. These findings suggest that, apart from autocatalytic processing, the protease activity of PCSK9 is not necessary for LDL-R regulation.

Novel Domain Interaction Regulates Secretion of Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) Protein

“…Elegant studies have shown previously that coexpressed PD trans-acts on PD-deficient PCSK9 and enables the latter to be secreted (27,28). We clearly confirmed these results in this study.…”

“…Fig. 2A shows that deletion of the entire PD protein abolished both the secretion and LDLR-reducing activity of PCSK9, consistent with previous reports (27,28). Separate deletions of PD residues 31-40, 41-50, and 51-60 did not significantly affect PCSK9 processing, secretion, or function, except for a modest effect of ⌬51-60 on autoprocessing.…”

“…Trans-Acting Effects of CD Deletion Mutants on PD-deficient PCSK9-It was reported previously that cotransfection of cells with a PCSK9 construct containing only PD and a construct missing PD caused secretion of a mature and fully functional chimeric PCSK9 protein (27,28). To investigate the trans-act- ing effects of different domains on PCSK9 secretion, we first coexpressed either ⌬CD or full-length PCSK9 with either ⌬PD or the CD fragment.…”

“…The structures of several other serine proteases have been solved in the mature form, where the prodomain is not part of the catalytically active protein (23)(24)(25)(26). Finally, biochemical data show that the catalytic activity of CA is not required for the LDLR-reducing activity in cells (27,28). This also makes PCSK9 different from other serine proteases (23)(24)(25)(26), and thus, its mode of action cannot be deduced from infamily comparisons.…”

Mutations and polymorphisms in the proprotein convertase subtilisin kexin 9 (PCSK9) gene in cholesterol metabolism and disease

“…Thus, PCSK9 functions as a chaperone to prevent LDL receptor recycling and/or target LDL receptors for lysosomal degradation. Furthermore, the ability of PCSK9 to degrade the LDL receptor is independent of its catalytic activity [McNutt et al, 2007;Li et al, 2007]. The role of PCSK9 serine protease activity seems limited to catalyzing the selfcleavage of pro-PCSK9, which is essential for PCSK9 secretion from liver cells and for the reduction of the number of LDL receptors [Zhang et al, 2007].…”